1. Field of the Invention
This invention relates generally to improvements in lifting devices and, more particularly, but not by way of limitation, to devices utilized to lift blow-out preventers during oil and gas well drilling operations.
2. Brief Description of the Prior Art
In the drilling of an oil or gas well, it is common for the drilling operation to be carried out from a drilling platform which forms a portion of the drilling rig and which is located some distance above the well proper; that is, the hole formed in the earth's surface by a drill. The drilling platform includes a rotary table which is mounted within a circular aperture in the floor formed by the drilling platform and means are provided for rotating the rotary table. The rotary table similarly has a central aperture and means are provided on the rotary table for gripping a drill string which extends through the rotary table so that the drill string can be rotated via the rotary table. The drill string is supported during drilling by a travelling block mounted on the drilling rig and positionable above the rotary table.
A casing, through which the drill string passes, is mounted in the well proper and it is common practice to bolt one or more blow-out preventers to the top of the casing, about the drill string, to close in the well should the drill penetrate a stratum in which gas is trapped under high pressure. These blow-out preventers are often massive structures; for example, blow-out preventers weighing in excess of 45 tons are used in drilling some wells. Such massive blowout preventers are also of considerable length, often extending from the casing to within a short distance of the bottom of the drilling platform.
At times, during the drilling of a well, it becomes necessary that operators of the drilling rig have access to the interior of the casing at the top of the casing. To provide access to the interior of the casing, at the top thereof, the blow-out preventer is unbolted from the casing and lifted therefrom. In some operations involving lifting of the blow-out preventer, the drill string will remain supported by the travelling block above the rotary table during the lifting operation and during the subsequent reattachment of the blow-out preventer to the casing.
In the past, it has been common practice to lift the blow-out preventer by means of hydraulic jacks suspended from the bottom of the drilling platform. The use of such jacks entails two serious problems. It is a time-consuming job to suspend the jacks from the drilling platform and connect the jacks to the blow-out preventer so that the job increases the cost of drilling a well. Moreover, the length of jacks suitable for lifting a massive blow-out preventer is not inconsiderable so that the attachment of the jacks to the blow-out preventer has, in the past, often been at lower portions of the blow-out preventer. The result is that the suspension of the blow-out preventer by jacks which, in turn, are suspended from the bottom of the drilling platform, is unstable. That is, the possibility exists that a blow-out preventer so suspended will topple to one side with considerable damage to drilling equipment and with considerable danger to persons working in the vicinity of the top of the casing.
It has also been common in the past to mount pulley systems, known as snatch blocks, to the underside of the drilling platform and lift a blow-out preventer via cables that pass from the blow-out preventer, through the snatch blocks, to winches mounted on trucks. The use of snatch blocks suffers from essentially the same two problems that make the use of jacks troublesome. It is a time consuming job to mount the snatch blocks and the entire system, once the lifting of the blow-out preventer commences, is unstable. A heavy blow-out preventer can result in the trucks being drawn toward the well rather than in the blow-out preventer being lifted. Since the blow-out preventer is unbolted from the well casing when the lifting commences, any movement of the trucks can result in toppling of the blow-out preventer. Thus, the use of snatch blocks and trucks leaves no room for error in blocking the trucks against movement. Thus, as with the use of jacks to lift a blow-out preventer, the use of snatch blocks is both time consuming and hazardous.